The present invention relates to a parking device for a blowout preventer aboard a drilling station such as a rig, platform or vessel.
When drilling for oil from floating drilling platforms or vessels, one is forced to use a so-called "blowout preventer" (BOP) or some sort of valving arrangement in order to be able to prevent any gas and oil found under pressure from freely and incontrollably "blowing" or escaping out into the sea and the consequences this will have. Together with the drilling mud in the drilled hole the BOP can control and prevent any undesirable flow out of the hole by either partly closing off the flow or by cutting the drill string and simultaneously sealing the hole completely.
As the depth of the holes drilled increases, the BOP valve will also increase in size and weight. Blowout preventers exist which weigh about 170 tons and have a height of 14 m.
During installation of the BOP, it is necessary to transport it from its parking place on the main deck (cellar deck) to a position beneath the drill deck (drill floor), where it can be attached to the drill string and be lowered by means of the drill string down to the sea floor. This operation today presents a problem because the clearance between the main deck and the lowest point of the drill floor is not sufficient to allow the BOP to be transported freely in one piece from its parked position to a position below the drill floor where it can be attached to the drill string and lowered down to the sea floor.
During lowering by means of the drill string, the BOP is guided down to an exact position on a foundation which has been lowered down and cemented to the sea floor. The guiding operation is accomplished by four guiding wires, each running through one of four tubes mounted in the four corners of the BOP.
The method presently used is to transport the BOP in two parts. The lower part of the BOP is first-lowered down below the main deck where it is fixed. The upper part is then transported in place and attached to the top of the lower part, so that the BOP can be lowered as a single unit by means of the drill string.
For this purpose various types of traversing cranes are used which are mounted below the drill floor and which can run longitudinally on the platform and thereby transport the two parts of the BOP to the desired position below the drilled floor. These cranes are often equipped with an extra crab having hoisting machinery for performing service on the BOP.
This method for transporting, storing and mounting the BOP is both cumbersome and time-consuming and allows the BOP to be easily subjected to damage. In addition, it is very difficult to give the BOP a functional test before it is mounted. Further problems arise when internal parts of the BOP need replacement, repair or maintenance.